In the exploration and development of oil and natural gas and coal deposits, during well drilling operations, drilling mud is circulated down through a hollow drilling string (drill stem) of a down hole pipe to both cool and lubricate the drill bit, as well as bring to the surface the formation cuttings from the hole. The mud is recovered by passing it up around the outside of the drill bit and the rotating drill stem which powers the bit. During its circulation through the drill hole, the mud entraps various types of gases and vapors, including any hydrocarbons which are present in the strata drilled through. As the mud is recovered from the hole, the entrapped gases are liberated from the mud and measurements are made to sense the presence of hydrocarbon gases and vapors and to determine the chemical composition of the gases and vapors. This information is useful in determining whether or not oil, natural gas or coal accumulations are located in strata penetrated by the drilling bit.
Hydrocarbon gases and vapors which are entrapped in drilling mud may be composed of a mixture of various types of alkanes (C.sub.n H.sub.2n+2) including methane, ethane, propane, n-butane, iso-butane, pentane, hexane, heptane, etc., which are typically present in continuous underground commercial reservoirs of oil, natural gas or coal. An accurate estimation of the presence of oil or natural gas requires not only the detection of the presence of hydrocarbon gases and vapors, but also the proportions of the types of constituent gases composing the hydrocarbon gas mixture.
The detected presence of methane gas indicates merely that some type of decayed organic material is present such as in a discontinuous reservoir usually referred to as a "pocket". However, such information may be useful in searching for coal seams, where the detection of methane during bore hole drilling at appropriate depths may mean the discovery of a useful deposit of coal. Although the presence of methane may also provide an initial indication of an oil or natural gas find, the presence of heavier molecular weight hydrocarbons provides an important confirmation of such a find, and may prevent a misleading conclusion associated with non-commercial presence of pockets of swamp gas. The heavier C.sub.2 -C.sub.10 hydrocarbons are not commonly found in living organisms and recent geological sediments but form a significant part of many commercial natural gas and crude oil deposits. The source of the carbonaceous material which generates these higher alkanes is an intermediate, polymeric material known as kerogen, which is a conglomeration of various chain hydrocarbons. The detection of heavier molecular weight hydrocarbon gases and vapors, such as butane and propane, signifies that an oil or natural gas reservoir may have been penetrated by the drill bit.
In the past, analysis of the composition of the hydrocarbon gas mixture entrapped in drilling mud has been carried out principally by use of a gas chromatograph. A gas chromatograph consists basically of a column filled with various types of packing material to separate the components of the hydrocarbon gas recovered from the drilling mud. As a sample of the hydrocarbon gas is passed through the tube, the different components of the gas exit the tube at different times depending on the boiling point of the components and their affinity to the different types of packing material. The discharge of the constituent gases from the column may be detected by changes in the electrical conductivity of a filament disposed at the discharge end of the column. The conductivity of the filament is dependent on the composition of the gas passing over the filament. Alternatively, a hydrogen flame may be located at the discharge end of the column, which flame creates ions which are sensed by appropriate instrumentation.
Gas chromatographs have operating characteristics which limit their usefulness in mud logging. The chromatographs do not operate properly when very large concentrations of hydrocarbon gases are present within the mud system. Also, due to the time span needed to analyze gases, the gases are sampled only on an intermittent batch basis. During the time interval between two sample batches, the drill bit may have already passed through a potentially productive reservoir horizon known as the "pay zone" where a commercial hydrocarbon accumulation may be entrapped. Thus, the gas chromatograph may miss the "pay zone" entirely. In addition, many types of gases recovered from oil and natural gas wells, such as hydrogen sulfide are corrosive thereby not only limiting the useful life of a chromatograph electrical filament, but also making it difficult to calibrate the detector. The hydrogen flame type detector requires the use of hydrogen gas which frequently is not conveniently available at remote drilling sites.
It is a principal object of the present invention to provide a method and apparatus for accurately, quickly and safely detecting, identifying and measuring the constituent hydrocarbon gases removed from drilling mud. It is known that certain frequencies of electromagnetic radiation in the infrared spectral region are highly absorbed by gases while other frequencies of infrared radiation are not absorbed or are only weakly absorbed. The highly absorbed frequencies correspond to the natural vibrational frequencies of the atoms of the gas molecules. In the present invention, lasers, which emit narrow line-width infrared radiation at frequencies corresponding to the absorption frequencies of hydrocarbon gases and vapors, are used to detect specific components in the gas samples removed from the drilling mud. The energy absorbed by the gas sample is measured at particular laser frequencies by photoacoustic absorption techniques to identify and quantify the individual gas components thereof.
In the past, lasers have been proposed for geologic exploration. U.S. Pat. No. 4,247,770 discusses the use of a laser carried by an aircraft to conduct aerial mineral surveys. The laser directs a pulsed beam at a point on the earth's surface to vaporize the material at the point of contact and generate an atomic emission spectra characteristic of the vaporized material. A spectrometer carried in the aircraft collects and analyzes the resultant spectra to determine the type and quantity of chemical elements present in the vaporized sample. A second, lower power laser may be then used to illuminate the vaporized material produced by the first, high power laser to generate the Raman spectrum of the vaporized material before it diffuses. After the vaporized material has diffused, the second laser may be used to illuminate the earth surface which has been freshly cleared by the high power laser beam to yield a fluorescent spectrum which also can be analyzed for mineral content.
Helium-neon laser systems have been used to measure methane. In an article by Grant et al. titled "Laser System for Global Detection of Natural Gas", published by Jet Propulsion Laboratory, Pasadena, CA., Second Quarterly Report, Apr. 20, 1981, GRI Contract No. 5080-352-0327, NASA Contact NAS7-100, the feasibility of utilizing lasers for the remote detection of leaks associated with storage, transportation and delivery of natural gases was explored. A system is proposed which utilizes a pair of helium-neon lasers to emit a first laser beam at a frequency strongly absorbed by methane gas and a second laser beam at a frequency weakly absorbed by methane gas. The extent to which the two laser beam frequencies are absorbed is measured by an InSb detector. A drawback of this particular system is that for accurate measurements, the detector must be maintained at a low temperature of approximately 77.degree. Kelvin. The cooling system needed to maintain the detector at such a low temperature is not only complicated, but also expensive to construct and maintain. Furthermore, this approach only yields quantitative data relating to the methane content of the gas sample. Specific information regarding the heavier hydrocarbon composition of the mixture is not obtainable from the two frequency helium-neon laser approach.
Gerritsen in an article titled "Methane Gas Detection Using a Laser", published in American Institute of Mining Engineers, Vol. 235 at 428 (1966), also discusses the use of a dual frequency helium-neon laser system to sense the presence of methane gas. A lead sulfide photoconductor is used to measure the extent to which the methane gas absorbs the energy of the laser beams.
R. M. Russ, Jr. in his masters thesis submitted at the Massachusetts Institute of Technology in June, 1978, describes another helium-neon system for measuring methane in the vicinity of a liquified natural gas spill. The system involves an etalon technique for deriving two separate analysis frequencies from a single helium-neon laser.
All of the laser systems discussed above, except the one described in the '770 patent, are intended only for methane detection. They cannot provide quantitative information regarding other hydrocarbon gases and vapors such as propane and butane. The laser system used in the '770 patent excites atomic emission, atomic Raman and fluorescences spectra and thus does not involve the infrared spectra employed in the present invention.